The present invention relates to packaging films, and more specifically to packaging films useful in vacuum packaging applications, and compositions for use in those films.
Vacuum packaging, and particularly vacuum skin packaging (VSP) has become an increasingly attractive way of packaging fresh red meats. The final package presents a tight fitting, clear package which protects the food article from the external environment. However, the demands imposed on the packaging material used in vacuum packaging, and especially in vacuum skin packaging, are high.
Three particular characteristics which are especially desirable in vacuum skin packaging applications, especially in the packaging of fresh beef, pork, broiled and browned pork, shingled turkey breast, and other meat items, are implosion or breakage resistance, forming ability and shelf life.
Generally, products that have cavities or undercuts are especially prone to implosions or leakers created by failure of the film during the vacuum skin packaging process. Using some materials, package failures as high as 60% have occurred. Thus, in actual practice, conventional films are sometimes broken when subjected to the high stretch ratios and particular projections, undercuts or recesses present on the material being packaged.
Good forming ability is also highly desirable in VSP applications to ensure that the heated film adequately conforms to the shape of the packaged product.
At the same time, high oxygen barrier characteristics are required in packaging material where the product to be packaged is sensitive to and degrades in the presence of oxygen. Fresh red meat products in particular typically require packaging materials with high oxygen barrier properties in order to ensure adequate or extended shelf life during storage, distribution and retail display under high as well as low humidity conditions.
The vacuum skin packaging process itself is now well known in the art. The packaging material generally comprises a top web and a bottom web which are each sent to the packaging station. The meat or other food to be packaged is placed onto the bottom web before the packaging station. The upper web comprises a film of a thermoplastic material which is optionally preheated and then fed to the packaging station and over the product on the bottom web. There it is usually heated by contact with a heated member, for instance the inner surface of a "dome". The space between the top and bottom webs around the food is then evacuated and the top web is allowed to come into contact with the bottom web and with the food. The top web may be held against the dome for instance by vacuum pressure which is released when it is desired to allow the top web to come into contact with the bottom web. Sealing of the top and bottom webs is achieved by a combination of heat from the dome and pressure difference between the inside of the package and the outside atmosphere and can be aided by mechanical pressure and/or extra heating. The heat that is supplied in the process is merely to allow the web to form and take up the form of the food product being packaged.
The implosion resistant films as described for example in U.S. Pat. No. 4,927,691 (Bekele) offer improved implosion resistance and resistance to bridging, good formability, and good overall oxygen barrier properties. It was nevertheless found desirable to provide a film with good implosion resistance, as well as good forming ability, which had improved oxygen barrier properties at relatively high humidities, where ethylene vinyl alcohol copolymer is especially susceptible to degradation in barrier properties as a result of the presence of moisture. This property of EVOH is well known in the art.
As taught for example in the brochure Engineering Resins produced by Emser Industries, amorphous nylon such as the Grivory.TM. G21 resin has excellent oxygen barrier properties at higher humidity conditions, and at 100% relative humidity is actually superior in oxygen barrier to EVOH resins. However, use of the amorphous nylon alone in multilayer films offers poor implosure resistance, unacceptable forming ability, and unacceptable oxygen barrier properties under low humidity conditions. This is especially true for the packaging of products such as fresh red meats in a vacuum or vacuum skin package process.
The inventor has found that by using both an amorphous nylon and an oxygen barrier material such as ethylene vinyl alcohol copolymer, very good oxygen barrier properties are obtained in the resulting film under both low and high humidity conditions.
In applications where good implosion resistance and forming ability is also required, the inventor has also found that these properties in the final film can be greatly enhanced by blending the amorphous nylon layer or layers of the multi-layer film with a low modulus polymeric material, more preferably a low modulus nylon (polyamide or copolyamide) such as nylon 6, 12; nylon 12; nylon 11; nylon 6, 66; nylon 6, 69; and nylon 610.
In applications where even better oxygen barrier performance at relative high humidities is desired, the most preferred modifiers for the amorphous nylon, i.e. blending materials, are nylon 6, 12; nylon 12; and nylon 11. This last group of materials is more moisture resistant, i.e. absorbs less moisture and transmits less moisture than many other nylon materials. In film where the primary oxygen barrier material such as EVOH is sandwiched between layers of an amorphous nylon blended with one of the above modifiers, especially the moisture resistant modifiers, moisture on the inside or the outside of a package formed from the film is substantially prevented from reaching the EVOH layer. This of course helps to prevent the oxygen barrier of the properties of the EVOH layer, and the overall film, from degrading in the presence of the high humidity environment.